The present invention relates to a magnetic brush developing apparatus for developing latent electrostatic images on an image bearing surface with a magnetic developer to obtain toner images.
A magnetic brush developing apparatus developed in recent years includes a developing sleeve rotatable in a direction opposite to the direction of transport of a magnetic developer, a magnetic roller disposed within the developing sleeve and rotatable at a high speed in the same direction as the developing sleeve, and a scraper by which the developer conveyed on the peripheral surface of the developing sleeve is scraped off the peripheral surface.
Such a magnetic brush developing apparatus will be described below more specifically with reference to FIGS. 1A and 1B. In this apparatus, a magnetic developer is magnetically attracted to the peripheral surface of a developing sleeve 1 by the magnetic action of a magnetic roller 2 within the sleeve 1 to form a magnetic brush 3 of the developer. As the magnetic roller 2 rotates at a high speed in the direction of an arrow a, the magnetic brush 3 is conveyed on the peripheral surface of the developing sleeve 1 in the direction of the arrow c and brushes against the surface of a latent electrostatic image bearing member 4, for example an electrophotographic photoconductive drum, in a developing zone 5, whereby the latent electrostatic image on the member 4 is developed to a visible image in the zone 5. The developer used for developing the latent image by brushing against the surface of the image bearing member 4 in the zone 5 is conveyed further in the direction of the arrow c, is thereafter scraped off of the surface of the sleeve 1 by a scraper 6 and falls away from the scraper due to gravity.
Repeated experiments using such a magnetic brush developing apparatus have been conducted, and it has been found that the developer is liable to stagnate in the vicinity of the scraper 6, which gives rise to various problems. This will be described in greater detail. When the apparatus is used continuously for a prolonged period of time for developing latent electrostatic images, the developer stagnates in the vicinity of the scraper 6 and progressively forms an accumulation 7 of developer at the forward end portion of the scraper 6 adjacent the developing zone 5, as shown in FIG. 1B. As a result, movement of the developer in the developing zone 5 is impeded, causing a portion of the developer forming the accumulation 7 to come into contact with the surface of the image bearing member 4 in an area other than the zone 5. This produces a distinct fog on the visible image, i.e. the developed toner image, or removes a portion of developer from the accumulation 7, causing the developer particles to escape from the developer container through the clearance between the open end 8 of the developer container and the surface of the image bearing member 4. The developer particles will scatter and, stain the developing apparatus in the neighborhood of the clearance. It has been found that such difficulties are especially great when the magnetic developer used is a mixture of magnetic carrier particles of reduced size and toner particles having electrical insulating properties.
An analysis appears to indicate that the chief cause for the accumulation of developer 7 is the following. When developer particles are scraped off the peripheral surface of the developing sleeve 1 by the scraper 6, the force acting on the developer particles to convey them along the guide surface of the scraper 6 in a direction away from the peripheral surface of the sleeve can be regarded as the sum of:
(i) The rolling force of the magnetic particles themselves which are contained in the developer and placed in a rolling motion by the rotation of the magnetic roller 2. PA1 (ii) The pushing force exerted on the above-mentioned developer particles by those conveyed on the peripheral surface of the sleeve.
The rolling force is attenuated greatly as the developer particles scraped off by the scraper 6 are conveyed along the guide surface of the scraper 6 and thereby carried away from the magnetic roller 2. On the other hand, the pushing force is not sufficiently great to convey the scraped-off developer particles smoothly along the guide surface by pushing them from behind, since the developing sleeve 1 is rotated in a direction opposite to the direction of transport of the developer. Accordingly the overall force which acts to convey the scraped-off developer particles along the guide surface of the scraper 6 in a direction away from the sleeve surface is very small. This is considered to be responsible for the stagnation of developer which is likely to occur in the vicinity of the scraper 6 and which leads to the progressive accumulation of developer 7 in the neighborhood of the end portion of the scraper and the developing zone.